Page 51 - Basic PD Theory

PD Investigations

* Semicon/Grading Interface
Failure mechanism Polarity Load Effect Temperature Effect Phase Location
Interface problems Positive None Direct 225°

Symptoms Detection Tests Machine types
Partial discharge, white PD, visual inspection, tan δ, power factor,
powder, ozone ozone monitoring Air-cooled with stress control paints

Slot discharge is described in Section 1.3.2.1. The PD pattern for electrical slot discharge is very similar to coil/bar movement
with positive PD near 225° predominating and sometimes a direct temperature effect. Since electrical slot discharge can occur before
windings become loose, there is no noticeable load effect for electrical slot discharge. Electrical slot discharge can be very intense
and in fact be a source of damage itself, for example, it can cause core damage.
Deterioration from surface PD sources is usually a slowly developing and long-term ageing mechanism,
unless accompanied by a direct load effect (see Section 6.6.3.1), which usually indicates coil/bar movement
and is especially a problem for thermoset windings.

* Electrical Slot Discharge
Failure mechanism Polarity Load Effect Temperature Effect Phase Location
Electrical Slot Discharge Positive None Direct 225°

Symptoms Detection Tests Machine types
Partial discharge, white PD, visual inspection, ozone monitoring, Air-cooled machines
powder, ozone coil-to-ground resistance, side clearance

Perhaps the most influential, yet unpredictable, impacts of ambient conditions to surface PD are the effects of variances in
ambient humidity on air-cooled machines. Humidity can cause the electrical breakdown stress of the air to decrease, and
therefore cause an increase in surface PD. On the other hand, humidity can also impact the electrical tracking of surface PD,
and therefore cause a decrease in PD. Thus, in some circumstances, the PD may go up (direct effect), while in others it would go
down (inverse effect) with increases in relative humidity. In some circumstances, the humidity effect has been known to cause the
PD to fluctuate by as much as 300%. Since each scenario is different, it is extremely important the ambient humidity be
recorded at test time so trends can be properly evaluated and decisions are not made about upward trends that occur because of
humidity effects and not changes in winding condition. Present experience indicates humidity only affects surface activity [11].

7.2 Endwinding Discharges
Pulses clustered at positions of the phase cycle other than 45° and 225° may be contingent upon other voltage references, such
as phase-to-phase voltage, phase-to-ground voltage from another phase, excitation, poor contact sparking or surface corona.
There may be some pulse-shifts at other angles, but these are infrequent. Since the PHA plot only shows negative pulses
occurring between -15° and 135° and positive pulses between 165° and 315°, a quick way to determine whether or not the pulses
are classic or non-classic is to compare the PHA results to the phase-resolved results. Pulses on the phase-resolved plots that are
not indicated on the PHA plots are indications of non-classic PD occurring.

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